Looks like you could use amd64 or x64.
Here's an excerpt from the vcvarsall.bat:
if /i %1 == amd64 goto amd64
if /i %1 == x64 goto amd64
So as you can see both options go to the same spot.
I also was getting the same warning with the windows,5.1 set in build.bat. The article
msdn.microsoft.com/en-us/library/fcc1zstk.aspx (the one you mentioned) after some closer examination shows to use:
5.01 for 32 bit
5.02 for 64 bit
The odd thing is it looks like you can use 5.1 or 5.2 to mean the same thing.
So if you want to clear that warning you need to run from a command prompt that has run vcvarsall.bat x86